Barrel nut with stress reduction features

ABSTRACT

A barrel nut with features for reducing tensile stresses under heavy load within the barrel nut has a partial-cylindrical body having a first planar end surface and a second planar end surface. A threaded bore extends through the partial-cylindrical body with a central axis substantially parallel to the first planar end surface and the second planar end surface. At least one groove is formed in each of the first planar end surface and the second planar end surface, the groove having a rounded surface extending at least a part of a distance between a curved upper surface of the partial-cylindrical body to a bottom surface thereof in a direction substantially parallel to the central axis of the threaded bore.

RELATED PATENT APPLICATION

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 14/469,640 filed on Aug. 27, 2014.

TECHNICAL FIELD

This disclosure generally relates to barrel nuts and more particularlyto barrel nuts adapted for attaching heavy structural members inhigh-tensile applications.

BACKGROUND

In general, a nut is a type of fastener with a threaded hole extendingthrough the fastener that is mated with a bolt having an exterior threadto fasten two or more parts together. The combination of the frictionbetween the nut threads and the bolt threads, a slight stretch of thebolt, and compression of the parts holds the parts together. The bolt isunder a constant tensile stress called the preload. The preload pullsthe nut threads against the bolt threads, and the nut face against abearing surface of one of the parts, with a constant force, so that thenut cannot rotate without overcoming the friction between thesesurfaces. In many applications, nut-and-bolt joints are subjected tovibration, which if intense enough, can cause the preload to increaseand decrease with each vibration cycle, and may cause the nut to becomeloose, crack or rupture.

Barrel nuts are specialized nuts, commonly used in aerospace, automobileand other applications that require high torque to bolt parts together,such as hanging aircraft engines from wings and attaching wings or tailsto an aircraft fuselage, or in applications where access to the nut islimited. A barrel nut is typically shaped like a round slug or acylinder. Some barrel nut designs incorporate a partially ‘flattened’surface to form, for example, a semi-cylindrical cross section. Barrelnuts have a threaded hole extending through the slug in a directionperpendicular to the length of the slug. In use, barrel nuts typicallysit inside a hole through a first part and a bolt is threaded into thebarrel nut from outside, passing through the second part that is to bebolted to the first part. Barrel nuts are preferred over a standard nutand bolt, when access is limited or not available, because they do notrequire any modification of the second part or additional materials,thus reducing weight and providing manufacturing and serviceefficiencies.

An exemplary aircraft field of use for a barrel nut is shown in FIGS. 1Aand 1B. An aircraft engine mount 10 is employed to attach an aircraftengine 12 to a strut 14 in an aircraft wing (not shown). The enginemount 10 is a structural element having cylindrical bores 16 extendingthrough the engine mount 10 in a horizontal direction with a centralaxis H. Bolt holes 20 are positioned through a top surface 22 of theengine mount such that the bolt holes 20 extend into the cylindricalbores 16 with a central axis V perpendicular to the central axis H ofthe cylindrical bores 16. Barrel nuts 18 are positioned within thecylindrical bores 16 in alignment with the bolt holes 20 to receivebolts 24, which may extend through holes in the strut 14 to attach thestrut 14 to the engine mount 10. Once inserted into the cylindricalbores 16, access to the barrel nut 18 is limited. In use, when load isapplied, the barrel nut 18 is exposed to stresses from the weight of theengine 12 and movement/flight of the aircraft.

SUMMARY

An improved barrel nut with features for reducing tensile stresseswithin the barrel nut during use is disclosed. The improvementseliminate cracking of the barrel nut under heavy load applications inareas around a threaded hole in the barrel nut, and therefore eliminatethe need to use larger diameter bolts and barrel nuts to accommodateheavy loads and expensive re-design of mounting structures toaccommodate larger diameter barrel nuts for such applications.

The improved barrel nut comprises a partial-cylindrical body having afirst planar end surface and a second planar end surface and a threadedbore extending through the partial-cylindrical body with a central axissubstantially parallel to the first planar end surface and the secondplanar end surface. Each of the first planar end surface and the secondplanar end surface comprises at least one groove with a rounded surfaceextending at least a part of a distance between a curved upper surfaceof the partial-cylindrical body to a non-curved bottom surface thereofin a direction substantially parallel to the central axis of thethreaded bore.

In a first embodiment, the at least one groove in each of the firstplanar end surface and the second planar end surface is aligned with thecentral axis of the threaded bore such that the at least one groove ispositioned on a center line of each of the first planar end surface andthe second planar end surface. In a second embodiment, each of the firstplanar end surface and the second planar end surface comprises twogrooves extending in a direction substantially parallel to the centralaxis, and positioned equidistant from a center line of each of the firstplanar end surface and the second planar end surface, the center linebeing aligned with the central axis of the threaded bore. In otherembodiments, additional grooves may be added to each of the first planarend surface and the second planar end surface, and configured in otherarrangements to provide stress reduction features.

In all embodiments, the bottom surface of the partial-cylindrical bodycomprises a flange surrounding the threaded bore. The flange is raisedabove the bottom surface and extends from a circumferential edge of thethreaded bore to a side edge of the bottom surface of thepartial-cylindrical body such that the curved upper surface of thepartial-cylindrical body extends below the bottom surface in a centralarea having a length substantially equal to a diameter of the threadedbore to provide additional strength and stress-reduction features aroundthe threaded bore. In addition, the length of the threads in thethreaded bore extends from the bottom surface to a point below thecurved upper surface of the partial-cylindrical body such that adistance between the point and the curved upper surface is about 2% toabout 10% of a total length of the threads.

Related methods of making a barrel nut with stress reducing features arealso considered to be within the scope of the present disclosure. Otherobjects, features, and advantages of the various embodiments in thepresent disclosure will be explained in the following detaileddescription with reference to the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates an exemplary aircraft environment of use in whichthe improved barrel nut may be used.

FIG. 1B illustrates an engine mount in the exemplary aircraftenvironment of FIG. 1 in which the improved barrel nut may be used.

FIG. 2A illustrates a top, left end and front perspective view of afirst embodiment of the improved barrel nut.

FIG. 2B illustrates a front plan view of the improved barrel nut shownin FIG. 2A.

FIG. 3A illustrates a top, right end and front perspective view of asecond embodiment of the improved barrel nut.

FIG. 3B illustrates a bottom, right end and rear perspective view of thebarrel nut shown in FIG. 3A.

FIG. 3C illustrates a right end plan view of the barrel nut shown inFIG. 3A.

FIG. 4 illustrates a bottom, left end and rear perspective view of analternative embodiment of the improved barrel nut.

FIG. 5 illustrates a bottom, right end and rear perspective view ofanother alternative embodiment of the improved barrel nut.

FIG. 6 illustrates the steps of a method of reducing stress in a barrelnut.

FIG. 7 illustrates the steps of a method of mounting an aircraft engineonto an aircraft using the improved barrel nut.

DETAILED DESCRIPTION

In the following detailed description, various embodiments are describedto illustrate the general principles in the present disclosure. It willbe recognized by one skilled in the art that the present disclosure maybe practiced in other analogous applications or environments and/or withother analogous or equivalent variations of the illustrativeembodiments. For example, the improved barrel nut may be used in anyapplication or industry that requires high torque to bolt parts togetherand the ability to withstand stresses from heavy loads. It should alsobe noted that those methods, procedures, components, or functions whichare commonly known to persons of ordinary skill in the field of thedisclosure are not described in detail herein.

Several embodiments of the improved barrel nut 30 are shown in FIGS.2A-2B, FIGS. 3A-3C, FIG. 4 and FIG. 5, respectively. The improved barrelnut 30 comprises a partial-cylindrical body 32 having a first planar endsurface 34 and a second planar end surface 36, an curved upper surface38 and a non-curved bottom surface 40 that is substantially flattened. Athreaded bore 42 having a diameter 31 extends through thepartial-cylindrical body 32, from the curved upper surface 38 to thebottom surface 40. The threaded bore 42 has a central axis 44substantially parallel to the planes of the first planar end surface 34and the second planar end surface 36.

The barrel nuts 30 have a one-piece construction formed from metals,metal alloys, steel or other materials depending on the intendedapplication. A preferred material is nickel alloy. The barrel nuts 30may also be made from corrosion and/or heat resistant materials, or havesurface finishes with protective features. For example, a solid filmlubricant coating may be applied inside the threaded bore 42 to coat theentire thread and to the curved upper surface 38 and bottom surface 40to preclude galling by reducing friction between metal to metalcontacts, prevent corrosion, and allow repeated assembly/disassembly ofthe barrel nut joint. Examples of specifications for such solid filmlubricants are disclosed in the U.S. Military SpecificationMIL-PRF-46010G, “Lubricant, Solid Film, Heat Cured, CorrosionInhibiting,” NATO Code—S-1738, Aug. 10, 2000.

To reduce stresses in the partial-cylindrical body 32 around thethreaded bore 42, and to avoid cracks in the curved upper surface 38near the threaded bore 42, each of the first planar end surface 34 andthe second planar end surface 36 comprises one or more grooves 46extending in a direction substantially parallel to the central axis 44of the threaded bore 42. Because the partial-cylindrical body 32 issymmetrical and the threaded bore is positioned in the center of thepartial-cylindrical body 32, it is preferred that the number andposition of grooves 46 in the first planar end surface 34 is the same asthe number and position of grooves 46 in the second planar end surface36.

The grooves 46 should extend at least part of the distance between thecurved upper surface 38 and the bottom surface 40 of thepartial-cylindrical body 32. The grooves 46 may extend from the curvedupper surface 38 of the partial-cylindrical body 32 all the way to thebottom surface 40 thereof, as shown in FIGS. 2A-2B, FIGS. 3A-3C and FIG.4. Alternatively, the grooves 46 may extend only part of the depth ofthe partial-cylindrical body 32 from either the curved upper surface 38or from the bottom surface 40. FIG. 5 shows an example of a barrel nut30 having a groove 46 extending from the bottom surface 40 onlypartially into the partial-cylindrical body 32. The groove 46 may end atan inner end point 53 between the curved upper surface 38 and the bottomsurface 40. The inner end point is rounded with a radius equal to orgreater than the radius of the groove 46, as described below.

Each of the grooves 46 preferably has a rounded surface, which may havea circular or oval shape. If an oval shape is used, the oval may bepositioned either vertically or horizontally.

The grooves 46 preferably have a smooth surface with a generous radiusto minimize stresses in the partial-cylindrical body 32. The radius R ofthe grooves 46 is a function of the diameter D of the barrel nut 30. SeeFIG. 3C. The radius R is generally in the range of about 0.015 times thediameter D to about 0.15 times the diameter D (i.e., 0.015*D to 0.15*D).

In a first embodiment of the barrel nut 30 shown in FIG. 2A and FIG. 2B,a single groove 46 in each of the first planar end surface 34 and thesecond planar end surface 36 is aligned with the central axis 44 of thethreaded bore 42 such that the single groove 46 is positioned in themiddle of the first planar end surface 34 and in the middle of thesecond planar end surface 36. That is, a first horizontal distance Abetween a first edge 47 of the single groove 46 and an outer edge 48 ofthe curved upper surface 38 is equal to a second horizontal distance Bbetween a second edge 49 of the groove 46 and the outer edge 48. Themiddle of the first planar end surface 34 and the middle of the secondplanar end surface 36 is also identified as the center line 50 shown inFIG. 3B.

A second embodiment of the barrel nut 30 having two grooves 46 in eachof the first planar end surface 34 and the second planar end surface 36is shown in FIG. 3A-3C. As in the first embodiment, the grooves 46extend in a direction substantially parallel to the central axis 44 ofthe threaded bore 42 from the curved upper surface 38 to the bottomsurface 40. But, in this embodiment, the grooves 46 are positionedequidistant from a center line 50 in the middle of each of the firstplanar end surface 34 and the second planar end surface 36. The centerline 50 is aligned with the central axis 44 of the threaded bore 42.That is, a horizontal distance C between a center 51 of each of thegrooves 46 and the center line 50 is equal. Since thepartial-cylindrical body is symmetrical, the horizontal distance C oneach side of the center line 50 may be any distance provided thehorizontal distance C on each side of the center line 50 issubstantially equal. In the second embodiment, the distance between thethreaded bore 42 and the grooves 46 is greater than in the firstembodiment. The preferred distance E between the centers 51 of each ofthe grooves 46 is a function of the diameter D of the barrel nut 30. Thedistance E is generally in the range of about 0.25 times the diameter Dto about 0.5 times the diameter D (i.e., 0.25*D to 0.5*D).

In other embodiments, additional grooves 46 may be added to each of thefirst planar end surface 34 and the second planar end surface 36, andconfigured in other arrangements to provide stress reduction features.For example, referring to FIG. 4, three grooves 46 may be provided inthe first planar end surface 34 and the second planar end surface 36.Each of the three grooves 46 may have a radius R that is the same or thecentral groove may be provided with a small radius and the two outergrooves may be provided with larger radii. The radii R of the grooves inthis embodiment are also a function of the diameter D of the barrel nut30, and may be in the range of about 0.015*D to about 0.15*D. Thepreferred distance F between the centers 51 of each of the grooves 46 inthis embodiment is also a function of the diameter D of the barrel nut30. The distance F is generally in the range of about 0.125 times thediameter D to about 0.25 times the diameter D (i.e., 0.125*D to 0.25*D).

In other embodiments, such as the single groove embodiment shown inFIGS. 2A-2B, the groove 46 may be formed with a smaller radius R toprovide a larger distance between the groove 46 and the threaded bore42.

The bottom surface 40 of the partial-cylindrical body 32 in allembodiments preferably comprises a flange 52 surrounding the threadedbore 42, which adds more material near the center of thepartial-cylindrical body 32 to increase the strength of the barrel nut30 and to prevent the barrel nut 30 from rotating. The flange 52increases inertia and therefore the bending capability of the barrel nut30 at the location of maximum bending stress—in the curved upper surface38 near the threaded bore 42. It is preferred to add enough material toincrease strength without substantially increasing the weight of thebarrel nut 30.

The flange 52 is raised above the bottom surface 40 and extends from acircumferential edge 54 of the threaded bore 42 to a side edge 56 of thebottom surface 40 of the partial-cylindrical body 32, such that thecurved upper surface 38 of the partial-cylindrical body 32 extends belowthe bottom surface 40 in a central area 58. Referring to FIGS. 3B, 4 and5, the central area 58 of the curved upper surface 38 below the bottomsurface 40 has a length L substantially equal to a diameter of thethreaded bore 42 to provide additional strength and stress-reductionfeatures around the threaded bore 42. The flange 52 extends around theentire circumference of the threaded bore 42, and has two straight edges57 at the side edges 56 of the bottom surface 40 and two curved edges 60facing the first planar end surface 34 and the second planar end surface36. Referring to FIG. 2B, the flange thickness T and the distance Gbetween the flange and each of the first planar end surface 34 and thesecond planar end surface 36 are a function of the diameter D of thebarrel nut 30. The flange thickness T is between about 0.05*D to about0.10*D. The distance G is between about 0.036*D to about 0.1*D. Theradius of curvature of the curved edges 60 is also a function of thediameter D of the barrel nut 30, and is preferably in the range of about0.87*D to about 1.46*D.

The threaded bore 42 comprises a plurality of threads 43 having a totallength 43 a extending from the bottom surface 40 to a point 62 below thecurved upper surface 38 of the partial-cylindrical body 32. See FIGS. 2Aand 3A. The distance between the point 62 and the curved upper surface38 is preferably about 2% to about 10% of the total length of thethreads.

Referring to FIGS. 3B, 4 and 5, a nylon or polymide lockring 64 may beprovided between the threads 43 and the bottom of the threaded bore 42in the area of the flange 52.

The barrel nut 30 disclosed herein reduces peak stress below the yieldor deformation limit (“Fty”) for the highest expected preload. Thegrooves 46 in the barrel nut 30 effectively move the peak stresslocations away from the curved upper surface 38 near the top of thethreaded bore 42. Stress analysis tests performed on a baseline barrelnut without the stress reduction features disclosed herein and thebarrel nut 30 with the disclosed stress reduction features show that thestress reduction features in barrel nut 30 reduce stress in the curvedupper surface 38 near the threaded bore 42 by about 15%.

Methods of reducing stress in a barrel nut and mounting an aircraftengine onto an aircraft are shown in FIGS. 7 and 8, respectively. Themethod of reducing stress 100 comprises the steps 102, 104 of forming abarrel nut 30 comprising a partial-cylindrical body 32 having a firstplanar end surface 34 and a second planar end surface 36 and the step104 of forming a threaded bore 42 extending through thepartial-cylindrical body 32 with a central axis 44 substantiallyparallel to the first planar end surface 34 and the second planar endsurface 36. In step 106, at least one groove 46 is formed in each of thefirst planar end surface 34 and the second planar end surface 36. The atleast one groove 46 has a rounded surface extending at least a part ofthe distance between a curved upper surface 38 of thepartial-cylindrical body 32 and a bottom surface 40 thereof in adirection substantially parallel to the central axis 44 of the threadedbore 42.

The method 200 for mounting an aircraft engine to an aircraft comprisesthe step 202 of inserting a barrel nut 30, comprising apartial-cylindrical body 32 having a first planar end surface 34 and asecond planar end surface 36, a threaded bore 42 extending through thepartial-cylindrical body 32, and at least one groove 46 in each of thefirst planar end surface 34 and the second planar end surface 36extending at least a part of a length of the partial-cylindrical body 32in a direction substantially parallel to a central axis 44 of thethreaded bore 42, into a cylindrical bore 16 extending through an enginemount 10. In step 204, the barrel nut 30 is aligned within thecylindrical bore 16 with bolt holes 20 positioned through a top surface22 of the engine mount 10. In step 206, bolts 26 are inserted throughthe bolt holes 20 and into the threaded bore 42 in the barrel nut 30,and tightened to the desired preload.

Many other modifications and variations may of course be devised giventhe above description of various embodiments for implementing theprinciples in the present disclosure. It is intended that all suchmodifications and variations be considered as within the spirit andscope of this disclosure, as defined in the following claims.

1. A method of reducing stress in a barrel nut comprising: forming a one-piece partial-cylindrical body having a first planar end surface and a second planar end surface; forming a threaded bore through the partial-cylindrical body with a central axis substantially parallel to the first planar end surface and the second planar end surface; and forming at least one groove recessed into each of the first planar end surface and the second planar end surface extending at least a part of a length of the partial-cylindrical body in a direction substantially parallel to the central axis of the threaded bore.
 2. The method of claim 1, further comprising forming the at least one recessed groove to have a rounded surface.
 3. The method of claim 2, wherein the rounded surface has a radius in the range of about 0.015 to 0.15 times a diameter of the barrel nut.
 4. The method of claim 1, further comprising aligning the at least one recessed groove in each of the first planar end surface and the second planar end surface with the central axis of the threaded bore such that the at least one recessed groove is positioned on a center line in each of the first planar end surface and the second planar end surface.
 5. The method of claim 1, further comprising forming two grooves in each of the first planar end surface and the second planar end surface to extend in a direction substantially parallel to the central axis from a curved upper surface to a bottom surface of the partial-cylindrical body.
 6. The method of claim 5, further comprising positioning the two grooves equidistant from a center line of each of the first planar end surface and the second planar end surface, the center line being aligned with the central axis.
 7. The method of claim 6, wherein a distance between a center line of each of the two grooves is in a range of about 0.25 to 0.5 times a diameter of the barrel nut.
 8. The method of claim 1, further comprising forming in a bottom surface of the partial-cylindrical body a flange surrounding the threaded bore, the flange being raised above the bottom surface and extending from a circumferential edge of the threaded bore to a side edge of the bottom surface.
 9. The method of claim 8, wherein the flange at the side edge extends a curved upper surface of the partial-cylindrical body below the bottom surface in a central area, the central area having a length substantially equal to a diameter of the threaded bore.
 10. A method of mounting an aircraft engine to an aircraft, comprising the steps of: inserting a barrel nut into a cylindrical bore extending through an aircraft engine mount, the barrel nut comprising a one-piece partial-cylindrical body having a first planar end surface and a second planar end surface, a threaded bore extending through the partial-cylindrical body and at least one groove recessed into each of the first planar end surface and the second planar end surface extending at least a part of a distance between a curved upper surface of the partial-cylindrical body and a bottom surface of the partial-cylindrical body in a direction substantially parallel to a central axis of the threaded bore; aligning the barrel nut within the cylindrical bore with bolt holes positioned through a top surface of the aircraft engine mount; inserting bolts through the bolt holes and into the threaded bore in the barrel nut; and tightening the bolts.
 11. A method of reducing stress in a barrel nut comprising: forming a partial-cylindrical body having a first planar end surface and a second planar end surface; forming a threaded bore through the partial-cylindrical body with a central axis substantially parallel to the first planar end surface and the second planar end surface; forming at least one groove into each of the first planar end surface and the second planar end surface extending at least a part of a length of the partial-cylindrical body in a direction substantially parallel to the central axis of the threaded bore; and forming in a bottom surface of the partial-cylindrical body a flange surrounding the threaded bore, the flange being raised above the bottom surface and extending from a circumferential edge of the threaded bore to a side edge of the bottom surface.
 12. The method of claim 11, wherein the flange at the side edge extends a curved upper surface of the partial-cylindrical body below the bottom surface in a central area, the central area having a length substantially equal to a diameter of the threaded bore.
 13. The method of claim 11, further comprising forming the at least one groove to have a rounded surface recessed into each of the first planar end surface and the second planar end surface.
 14. The method of claim 13, wherein the rounded surface has a radius in a range of about 0.015 to 0.15 times a diameter of the barrel nut.
 15. The method of claim 11, further comprising aligning the at least one groove in each of the first planar end surface and the second planar end surface with the central axis of the threaded bore such that the at least one groove is positioned on a center line in each of the first planar end surface and the second planar end surface.
 16. The method of claim 11, further comprising forming two grooves in each of the first planar end surface and the second planar end surface to extend in a direction substantially parallel to the central axis from a curved upper surface to the bottom surface of the partial-cylindrical body.
 17. The method of claim 16, further comprising positioning the two grooves equidistant from a center line of each of the first planar end surface and the second planar end surface, the center line being aligned with the central axis.
 18. The method of claim 16, wherein a distance between a center line of each of the two grooves is in a range of about 0.25 to 0.5 times a diameter of the barrel nut.
 19. The method of claim 11, further comprising forming the flange with a thickness between about 0.05 to about 0.10 times a diameter of the barrel nut.
 20. The method of claim 11, further comprising forming the threaded bore with a plurality of threads having a total length extending from the bottom surface to a point below a curved upper surface of the partial-cylindrical body. 